This invention pertains generally to radar systems and particularly to such radar systems as those which incorporate quadrature demodulators.
It is known in the art that a limiting factor in the operation of conventional pulse Doppler or pulse compression radar systems is the accuracy with which intermediate frequency signals may be translated, or downconverted, to provide the requisite demodulated received signals for final processing. The accuracy of such translation, or demodulation, is of particular import when the final processing is to be carried out by using digital computation techniques. In such a case, because the demodulated received signals may most effectively be derived by any well known quadrature demodulation process, it is highly important that neither the demodulation process nor the conversion of the demodulated analog signals to complex digital numbers introduce errors in the demodulated signals. Unfortunately, however, imbalances existing in the circuitry of any practical quadrature demodulator result in significant errors in the demodulated signals. In particular, such imbalances cause unwanted frequency components to be generated in the frequency spectra of the demodulated frequency signals.
If, as in the case in any practical quadrature demodulator, imbalances between channels cannot be avoided then, obviously, the next best thing is to compensate in some way for the resulting change caused by such a demodulator in the frequency spectrum of any received signal being processed. Thus, in many cases, known "pilot pulse" calibrating techniques may be employed. According to a typical one of such techniques a test signal of fixed amplitude with a known frequency spectrum, i.e. a pilot pulse, is periodically passed through a receiver and the cumulative effect of all elements in the receiver (including a quadrature demodulator) on the frequency spectrum of the pilot pulse is observed. Adjustment of selected elements then may be effected to compensate for what may be considered an "average change in the frequency spectrum of the pilot pulse in passing through the receiver."
While any such pilot pulse calibrating technique obviously may be applied to a compensation procedure for error induced only in a quadrature demodulator in a receiver, it is equally obvious the resulting calibration will not be completely accurate for each frequency in any signal having a broad frequency spectrum. Because the error induced in any known quadrature demodulator is dependent upon both the amplitude and the frequency of the signals being demonstrated, it follows that any conventional pilot pulse calibration technique is not completely effective when a quadrature demodulator is used in a radar system processing signals which have relatively wide frequency spectra, such as a pulse Doppler or a pulse compression radar system.
In view of the foregoing it is a primary object of this invention to provide an improved method for calibrating a quadrature demodulator in a radar system such as a pulse Doppler or a pulse compression radar system.